Commercial low cost vehicle positioning and tracking systems represent large potential applications and markets. Commercial vehicle tracking is just one of the many applications of these systems. A commercial vehicle refers here to public service vehicles, such as those used by ambulance, fire, police, and transit departments, as well as to all classes of vehicles used in business and government service. An intelligent vehicle highway system is another application field. Intelligent vehicle highway systems apply computer, positioning, communications, and control technologies to integrate vehicles and highways in a coherent information network that facilitates the travel of individual vehicles, while optimizing traffic flow and increasing traffic capacity throughout the entire road system.
During the late 1980's, major efforts commenced through the entire world to develop and apply intelligent vehicle highway systems to reduce congestion, improve mobility and road transportation efficiency, enhance safety, conserve energy, and protect the environment. Although the taxonomy of intelligent vehicle highway systems is not yet fully consistent worldwide, the following six categories encompass virtually all elements of intelligent vehicle highway systems developed across the globe.
Advanced traffic management systems extend real-time computer optimization of traffic signal timing to the urban road network level as opposed to individual intersections or streets. This requires information on traffic conditions throughout the network in a real-time database that may also serve as an information source for dynamic route guidance in advanced traveler information systems-equipped vehicles.
Advanced traveler information systems keep drivers being informed of their location and provide route guidance to these selected destinations along with information on services such as lodging, food, fuel, repair, medical facilities, etc. The advanced traveler information systems permit communication between in-vehicle equipment and advanced traveler information systems for data on traffic conditions, diversion routes, alternative modes of transportation, etc. Although advanced traveler information system concepts originally centered on vehicular navigation and route guidance for drivers, new concepts of advanced traveler information systems include portable versions for use by pedestrians and multimodal travelers also.
Commercial vehicle operations include vehicle tracking and fleet management systems for commercial and emergency vehicles to improve operational efficiency and increase safety. They also include technologies such as automatic vehicle classification, weigh-in-motion, and communications among automated regulatory checkpoints so that intercity trucks may travel among different jurisdictions with minimal stopping.
Advanced Vehicle Control Systems apply additional technologies to vehicles to detect obstacles and adjacent vehicles, thus enhancing vehicle control by augmenting driver performance. Advanced vehicle control systems assist in the prevention of collisions for safer high-speed driving to increase roadway capacity, and they will eventually interact with fully developed advanced traffic management systems to enable automatic vehicle operation.
In addition to applying the above mentioned intelligent vehicle highway system technologies to public transportation systems, Advanced Public Transportation Systems have a strong focus on customer interface. Examples include onboard displays (e.g., for next stop, transfer information, etc.), real-time displays at bus stops, and smart card fare systems as well as ride share and high-occupancy vehicle information systems.
Advanced Rural Transportation Systems focus on issues and problems involving the development and application of intelligent vehicle highway systems to rural transportation. The major thrusts of Advanced Rural Transportation Systems include emergency communications and safety applications of intelligent vehicle highway system technologies.
Intelligent vehicle highway systems are still in the early stages of development, and, although numerous operational field trails are underway, relatively few actual applications of intelligent vehicle highway system technologies have been implemented.
Nowadays, there exist stand-alone operating positioning systems and ground proximity warning systems in civil aircraft. A positioning system is used to provide position, velocity, attitude, attitude rate information, and etc., for an aircraft flight control and management system. A ground proximity warning system is used to provide warning messages to prevent aircraft from inadvertently contacting with the ground or water.
Traditionally, positioning equipment in a civil aircraft generally employs an inertial navigation system and some radio navigation systems, such as a long range navigation system, a very-high-frequency omnidirectional range system, distance measurement equipment, tactical air navigation, and the newest global positioning system. Recently, integrated global positioning systems/inertial navigation systems have been the predominant navigation system in civil and military aircraft, replacing traditional navigation systems.
Generally speaking, an inertial navigation system comprises an onboard inertial measurement unit, a processor, and embedded software. The positioning solution is obtained by numerically solving Newton's equations of motion using measurements of vehicle specific forces and rotation rates obtained from onboard inertial sensors. The onboard inertial sensors consist of accelerometers and gyros, which, together with the associated hardware and electronics, comprise the inertial measurement unit.
The inertial navigation system may be mechanized in either a gimbaled or strapdown configuration. In a gimbaled inertial navigation system, the accelerometers and gyros are mounted on a gimbaled platform to isolate the sensors from the rotations of the vehicle, and to keep the measurements and navigation calculations in a stabilized navigation coordinated frame. Some possible navigation frames include earth centered inertial, earth centered earth fixed, locally level, with axes in the directions of north, east, down (of course, there is east, north, zenith or north, west, zenith, and locally level with a wander azimuth). In a strapdown inertial navigation system, the inertial sensors are rigidly mounted to the vehicle body frame, and a coordinate frame transformation matrix (analyzing platform) is used to transform the body-expressed acceleration to a navigation frame to perform the navigation computations in the stabilized navigation frame. Gimbaled inertial navigation systems can be more accurate and easier to calibrate than strapdown inertial navigation systems. Strapdown inertial navigation systems can be subjected to higher dynamic conditions (such as high turn rate maneuvers) which can stress inertial sensor performance. However, with the availability of newer gyros and accelerometers, strapdown inertial navigation systems are becoming the predominant mechanization, due to their low cost and reliability.
In principle, inertial navigation systems permit pure autonomous operation and output continuous position, velocity, and attitude vehicle data after initializing the starting position and initiating an alignment procedure. In addition to autonomous operation, other advantages of an inertial navigation system include the full navigation solution and wide bandwidth. However, an inertial navigation system is expensive and subjected to drift over an extended period of time. This error propagation characteristic is primarily caused by its inertial sensor error sources, such as gyro drift, accelerometer bias, and scale factor errors.
Generally, the accuracy of inertial navigation systems can be improved by employing highly accurate inertial sensors or by compensating with data from an external sensor.
The cost of developing and manufacturing inertial sensors increases as the level of accuracy improves. The advances in new inertial sensor technologies and electronic technologies have led to the availability of low cost inertial sensors, such as mechnical-electronis-micro-system inertial sensors. Mechnical-electronic-micro-system inertial sensors borrow processes from the semiconductor industry to fabricate tiny sensors and actuators on silicon chips. The precision of these new inertial sensors may be less than what conventional sensors achieve, but they have enormous cost, size, weight, thermal stability and wide dynamic range advantages over conventional inertial sensors.
The most obvious choice for implementing low cost, highly accurate, continuous positioning of a vehicle is to employ a low cost strapdown inertial system with the compensating of an external sensor. The global positioning system receiver is an ideal external sensor for an inertial navigation system.
The global positioning system is a space-based, worldwide, all-weather passive radio positioning and timing system which was developed and implemented over the course of the past two decades. The system was originally designed to provide precise position, velocity, and timing information on a global common grid system to an unlimited number of adequately equipped air, land, sea, and even space authorized users and civil users.
The global positioning system has three major operational segments:
Space Segment: The Space segment consists of a constellation of satellites (21 navigation satellites plus 3 active spares) in semi-synchronous orbit around the earth.
Control Segment: The control segment consists of one master ground control station and several other monitor stations with tracking antennas at accurately known positions throughout the earth.
User Segment: The User Segment is composed of the various kinds of end user with global positioning system receiving equipment.
The global positioning system user equipment comprises an antenna, a receiver, and associated electronics and displays, and receives signals from the global position system satellites to obtain a position, velocity, and time solution.
The global positioning system can provide Precise Positioning Service to authorized users, which is nominally within 15 meters Spherical Error Probable accuracy, and can provide Standard Position Service to civil users, which is limited to within roughly 100 meters (95% probability) by a number of error sources including ionospheric and troposheric effects and intentional degradation of the global positioning system signal, known as selective availability.
The global positioning system principle of operation is based on range triangulation. If the satellite position is known accurately via ephemeris data, the user can receive the satellite's transmitted signal and determine the signal propagation time. Since the signal travels at the speed of light, the user can calculate the measured range to the satellite. The actual range measurement (called the "pseudo range") contains errors because of a bias in the user's clock relative to the global positioning system reference time. Because atomic clocks are utilized in the satellites, their errors are much smaller in magnitude than the users' clocks. Thus, for three-dimensional position determination, and also to calculate the clock bias, a minimum of four satellites is needed to obtain a solution to the navigation problem. The velocity can be obtained by various methods, which basically amount to time differencing the pseudo ranges over the measurement time interval.
As with any other measurement system, a global positioning system contains a number of error sources, such as the signal propagation errors and satellite errors, including selective availability. The user range error is the resultant ranging error along the line-of-sight between the user and the global positioning system satellite. Global positioning system errors tend to be relatively constant (on average) over time, thus giving global positioning systems long-term error stability. However, the signals of the global positioning system may be intentionally or unintentionally jammed or spoofed, or the global positioning system receiver antenna may be obscured during vehicle attitude maneuvering, and the global positioning system signals are lost when the signal-to-noise ratio is low, and the vehicle is undergoing highly dynamic maneuvers.
The inherent drawbacks of a stand-alone inertial navigation system and a standalone global positioning system receiver show that a stand-alone inertial navigation system or a stand-alone global positioning system receiver can not meet mission requirements under certain constraints, such as low cost, long-term high accuracy, continuous output, high degree of resistance to jamming, and high dynamics.
In the case of integration of a global positioning system with an inertial navigation system, the short term accuracy of the inertial navigation system and the long term stability and accuracy of the global positioning system directly compliment each other. The global positioning system is fairly accurate but available at a slower data rate. The inertial navigation system data has low noise and is available at high data rates, but it is subjected to biases and drift that cause the errors to grow with time. The performance characteristics of the mutually compensating stand-alone global positioning system receiver and the stand-alone inertial system suggest that, in many applications, an integrated global positioning system/inertial navigation system, combining the best properties of both fields, will provide optimal continuous navigation capability. This navigation capability is unattainable in either one of the two systems alone.
The potential advantages offered by an integration of a global positioning system receiver with an inertial navigation system are outlined as follow:
(1) The integration smoothes out the random component in global positioning system observation errors, and can compensate the navigation parameter errors and inertial sensor errors of the inertial navigation system while the global positioning system signal is available, so that the inertial navigation system can provide more accurate position and attitude information during an extended period of time after the global positioning system signals are lost.
(2) The aiding of the signal tracking loop process of the global positioning system receiver with inertial data. This allows the effective bandwidth of the loops to be reduced, resulting in an improved tracking signal in a noisy environment while not sacrificing global positioning system signal dynamic tracking performance.
There are usual signal tracking loop bandwidth versus dynamic performance tradeoffs commonly encountered in signal tracking loop design of a global positioning system receiver, wherein noise effects increase with increasing loop bandwidth, while dynamic tracking errors increase with decreasing loop bandwidth. The integrated global positioning/inertial navigation system can mitigate the conflicting signal tracking loop bandwidth requirements, because the global positioning system signal acquisition and tracking processes are aided by inertial navigation data.
(3) An inertial navigation system can, not only provide navigation information when the global positioning system signals are lost temporarily, but also reduce the search time required to reacquire the global positioning system signal.
(4) The global positioning system enables and provides on-the-fly alignment of an inertial navigation system by the means of maneuvering, eliminating the static self-alignment pre-mission requirements, and improving the reaction of the inertial navigation system.
However, there are still some drawbacks in conventional integrated global positioning/inertial navigation systems as follows:
(1) Poor vertical measurement accuracy, which can not meet the requirement for precise terminal approach, landing, and collision avoidance in heavy traffic airspace.
(2) Insufficient reliability. When a low cost, low accuracy inertial navigation system is employed to integrate with a global positioning system receiver, long-term navigation accuracy is mostly dependent on the global positioning system. If global positioning system signals are lost for a short period of time, or if the malfunction of a global positioning system satellite occurs, the navigation accuracy diverges very fast.
Therefore, there is an urgent need to overcome these drawbacks to enhance aviation safety.
As aviation markets are extended, there are more and more emphases on aviation safety. Usually, one aircraft accident classification scheme includes:
(1) Controlled Flight Into Terrain PA1 (2) Loss of Control (caused by an aircraft malfunction) PA1 (3) Loss of control (caused by crew error) PA1 (4) Airframe structure or system failure PA1 (5) Mid-air collision PA1 (6) Ice/Snow PA1 (7) Fuel exhaustion PA1 (8) Loss of control (other) PA1 (9) Runway Incursion PA1 (1) Excessive descent rates while too close to the ground. PA1 (2) Excessive terrain closure rates (aircraft is descending too quickly or is flying toward higher terrain). PA1 (3) Excessive descent rates after takeoff. PA1 (4) Inadvertent descent after takeoff PA1 (5) Insufficient terrain clearance. PA1 (6) Flight into terrain at low altitude and not in approach and landing configuration. PA1 (1) The performance of an integrated positioning system and the ground proximity warning system is unattainable in the two systems alone. PA1 (2) The positioning accuracy of the present invention is higher than the conventional integrated global positioning system /inertial navigation system alone. PA1 (3) The system of the present invention has ability to detect the malfunction of the global positioning system satellite. PA1 (4) Prompt and accurate ground proximity warning message is available, due to more accurate positioning solution provided by the system of the present invention. PA1 (5) The system of the present invention reduces false ground proximity warning probability, due to a more accurate positioning solution provided by the system of the present invention. PA1 (6) Compared with the conventional enhanced ground proximity warning systems, in the system of the present invention, an external navigation system is not required to support the ground proximity warning solution. This is especially affordable for small commercial aircraft vehicle.
Since the beginning of powered flight, Controlled Flight Into Terrain-type airplane accidents have been a worldwide problem and an important accident classification. In these airplane crash accidents, a properly functioning airplane under the control of a fully qualified and certified crew, often in clouds or darkness, is flown into terrain or water or obstacles with no apparent awareness on the part of the crew. There is no airframe icing, no wind shear, no collision with other aircraft, and no loss of control. The Controlled Flight Into Terrain has received a number of studies and considerable attention since the early 1970's.
Since the Federal Aviation Administration mandated an independent Ground Proximity Warning System in 1974 for commercial turbojet/turboprop aircraft with more than 10 passenger seats which fly in U.S. airspace, traditional ground proximity warning systems have dramatically reduced the number of Controlled Flight Into Terrain accidents among airlines, by monitoring the aircraft 's height above ground as determined by a radio altimeter. There have been one to two aircraft Controlled Flight Into Terrain accidents a year in average since 1985. Before the mandate, it varied from seven to eighteen years.
The ground proximity warning system computer keeps track of the radio altimeter readings and other flight information and provides audible, visual, and meaningful warnings to aircrew when the aircraft flight status meets any of the following
Although traditional ground proximity warning systems have saved thousands of lives, it still fails to prevent all Controlled Flight Into Terrain accidents. Traditional ground proximity warning systems can not really tell what is ahead of the airplane. It can only look down, as the radio altimeter measures the distance from the airplane to the ground. The ground proximity warning system is designed to predict if a potential terrain problem lies ahead by tracking information from the recent past and projecting any trend into the future. This can warn a pilot when the airplane is flying towards terrain that slops upward, but it can not warn in time to avoid, for example, a sheer cliff or extremely steep slope. In addition, ground proximity warning systems have to be "desensitized" under certain conditions to prevent nuisance warnings. For instance, when wing flaps and landing gear are extended, the system is desensitized to prevent an unnecessary warning while the pilot intentionally flies the airplane toward the ground for landing.
The available improved ground proximity warning system, which is called the Enhanced Ground Proximity Warning System, is the result of an effort to further reduce the Controlled Flight Into Terrain risk. The enhanced ground proximity warning system uses a worldwide digital terrain database. Also, the enhanced ground proximity warning system adds two enhancements to the traditional ground proximity warning system:
(1) It can provide the flight crew a map-like display of nearby terrain.
(2) It sounds an audible alert approximately one minute's flight time or more away from terrain. Traditional ground proximity warning systems typically sound a warning from a few seconds to about 30 seconds from terrain, but average 10 to 15 seconds.
The enhanced ground proximity warning system computer uses information provided by the onboard navigation system and terrain database. The enhanced ground proximity warning system computer uses aircraft position, performance, and configuration data to calculate an envelope along the projected flight path of the aircraft and compares that to the terrain database.
Since the enhanced ground proximity warning system display can show nearby terrain, pilots are much less likely to continue flying toward that terrain.
In current developing degree of the positioning systems and the ground proximity warning systems, the positioning system and the ground proximity warning system are two separated independent systems, wherein another external positioning system is required to support the ground proximity warning system.
As above mentioned, conventional integrated global positioning/inertial navigation systems integrates the information from an inertial measurement unit and a global positioning system receiver to obtain improved navigation solution. Conventional ground proximity warning systems use the position information provided by the conventional integrated global positioning/inertial navigation systems or stand-alone global positioning systems or stand-alone inertial navigation system and the information from a radio altimeter and a baro altimeter and a terrain database to solve ground proximity warning problem. The process for both conventional integrated global positioning/inertial navigation systems and conventional ground proximity warning systems is implemented in software in microprocessors. As advances in speed and memory of microprocessors, it is possible to implement an integrated positioning and ground proximity warning system. Furthermore, an integrated process for position solution and ground proximity warning solution can provide better performance than an independent positioning system and an independent ground proximity warning system, because the information from the radio altimeter, the baro altimeter, and the terrain database which are employed by conventional ground proximity warning systems has potential capability for improving the accuracy and reliability of conventional positioning systems, and improved position information in turn improve the performance of conventional ground proximity warning systems.